Transduction barrier

Transduction barriers were protective shields surrounding Gallifrey and the Time Lords. These barriers could also be raised around Time Lord space stations, and TARDISes were normally unable to pass through them. (AUDIO: Trial of the Valeyard)

It took the disciples of Omega four generations to install the barriers, which were powered by the Eye of Harmony. They kept Gallifrey slightly out of phase with the rest of the universe, protecting the world from invasion by expelling anything foreign to Gallifrey, normally destroying the invasive force. There were at least five levels of intensity to the barrier. (AUDIO: Renaissance)

According to another account, there were four barriers. The First Barrier was a force field that could tear matter apart at a sub-atomic level. The Second Barrier was made of ideas, and refused to let anything pass because it didn't believe it was possible. The Third Barrier used reflection to counter any approach with an equal and opposite idea. The Fourth Barrier of Stubborn Philosophy refused to believe in the existence of any alien craft and would reiterate this until the enemy gave up. (PROSE: Doctor Who and the Krikkitmen)

The Time Lord's vault on Tersurus was protected by an outdated transduction barrier which the Master was able to break through using Goth's TARDIS. Before the barrier could be updated and the break-in retroactively undone, the Master killed the vault operators. (AUDIO: Dominion)

As he tracked the origin of a mysterious white energy that was consuming space, the Eleventh Doctor sailed down a micro-scale dimensional conduit, emerging on ancient Gallifrey, Knowing that this was supposed to be impossible, he suspected that the conduit was what had allowed his TARDIS to bypass the transduction barriers. (COMIC: The Lost Dimension)